It is a challenge to create a system for measuring the magnitude and direction of the static unbalanced forces in a tire or tire and wheel assembly resulting from the dislocation of the center of gravity of such tire from its intended location at the axis of symmetry and rotation. The task is complicated by several factors. First, the required accuracy and resolution is high, and the unbalanced moments to be measured may be extremely small. Secondly, such measurements must be made in the presence of very large forces arising from the weight of the tire or tire and wheel, which forces should preferably be bypassed around the measuring structure. This factor is further complicated by the fact that these static forces may be momentarily increased many times by the shock of loading the tire unto the measuring device or of clamping it in place. Third, for maximum accuracy and stability, it is preferable to have the sensing system at or close to the intended center of gravity of the object being measured, and therefore there is a limited amount of space available for the measuring structure.
It is common in the art of force or moment measuring to utilize electrical resistance strain gages bonded to the surface of a beam in such a fashion that the bending of the beam, which is proportional to an applied force or moment, produces a proportional change in the electrical resistance of the gage, which in turn produces an electrical signal proportional to the magnitude of the force being measured. Bending moments are commonly measured by resolving such moment into a force applied to a calibrated beam at a fixed lever arm distance from a point about which the system tries to pivot. Generally, however, such measurements do not achieve the theoretical degree of accuracy and reliability, because fabricating, space, frictional or other considerations necessitate compromises.
Accordingly, it is the principal object of the present invention to provide an improved system for measuring the unbalanced moments about two mutually perpendicular axes, such as those arising from the displacement of the center of gravity of a tire from its design point, and to achieve maximum stability and accuracy of results.